Water filtration systems designed for residential and commercial use have become increasingly popular. The popularity arises from the need to remove unwanted substances from input water to make output water safer for consumption in various end uses.
Two common water filtration systems are: 1) Systems that discharge product water into an enclosed pressure vessel against back pressure created by an air cell within the vessel (air-on-water system); and 2) Systems that discharge product water, in the absence of back pressure, into an enclosed pressure vessel and into a flexible water cell that can be compressed by a separate source of water to remove the product water from the vessel (water-on-water system).
Air-on-water systems are subject to the back pressure of the air cells which, essentially, reduces the pressure differential across the filtering portion of the system (e.g., a reverse osmosis membrane), thereby reducing the quality and quantity of filtered product water made in a given time. Product water quality particularly suffers if the product water is frequently drawn off and replaced in small quantities, as typically occurs in household systems that include a single filtering portion and a single storage vessel. Moreover, as the air cell-propelled water is emptied from the storage vessel, the air cell gradually loses pressure and the dispensing flow rate of the product water declines.
Most air cell systems include an automatic shut-off valve that stops feed water flow, and thus further production of slow flush waste water, when the storage tank is full and typically reaches 60%-70% of line pressure. This technique, while reducing waste, can result in reduced quantity and quality of the product water and its dispensing flow rate.
Water-on-water system can address many of the shortcomings of air-on-water system. Water-on-water system typically include a pressure vessel containing two water-filled compartments of approximately the same size. The physical separation between the compartments is movable or flexible so that water pressure in a first compartment influences the water pressure in the second compartment. Each compartment is accessed by different fluid sources so that one compartment can be filling while the other one is emptying. Thus, little or no pressure drop occurs across the compartments. Both compartments are pressurized, when product water is drawn out of the vessel. Both compartments are then depressurized when product water is filling one compartment and displacing water from the other compartment to drain.